The present invention relates to direct thermal and thermal transfer demand printers and specifically to direct thermal and thermal transfer printers for printing on tickets, tags, and pressure-sensitive labels. Some aspects of the invention also relate to printers using other printing techniques such as laser printing, LED printing, etc.
Direct thermal and thermal transfer printers are well known in the prior art. For thermal transfer printing on nonsensitized materials such as paper or plastics, a transfer ribbon coated on one side with a heat-transferrable ink layer is interposed between the media to be printed and a thermal printhead having a line of very small heater elements. When an electrical pulse is applied to a selected subset of the heater elements, localized melting and transfer of the ink to the paper occurs underneath the selected elements, resulting in a corresponding line of dots being transferred to the media surface.
For direct thermal printing on sensitized materials, no transfer ribbon is used and the heater elements act directly to produce chemical or physical change in a dye coating on the surface of the material. The balance of this disclosure discusses thermal transfer printing, but it should be clear that many aspects of the present invention apply equally to direct thermal printing, laser printing, LED printing, and perhaps others as well.
After each line of dots is printed, the material or printhead is repositioned to locate the printhead over an adjacent location, the transfer ribbon is repositioned to provide a replenished ink coating, and the selecting and heating process is repeated to print an adjacent line of dots. Depending upon the number and pattern of heaters and the directions of motion of the head and paper, arrays of dots can produce individual characters or, as in the preferred embodiment, successive rows of dots are combined to form complete printed lines of text, bar codes, or graphics.
Applications of such printers include the printing of individual labels, typically pressure-sensitive labels, tickets, and tags. Pressure-sensitive labels are commonly presented on a continuous web of release material (e.g., waxed paper backing) with a gap between successive labels. Tickets and tags may likewise be presented as a continuous web with individual tickets or tags defined by a printed mark or by holes or notches punched therein. Tickets and tags also may likewise be presented on a continuous web with individual tickets or tags defined by a printed mark or by holes, slits, or gaps punched therein.
An optical sensor may be used for the alignment of the printed image with the leading edge of each label. The optical sensor comprises an illumination source such as a light-emitting diode ("LED") or incandescent lamp, and a photo-detector such as a photo resistor, photo transistor, or photo diode. The illumination source and the photo detector typically, but without limitation, function at an infrared wavelength. In the preferred embodiment(s), the sensor is disposed through the, web so as to respond to the change in relative opacity of the backing and label materials, or to a hole or notch punched in the web. In other embodiments, the sensor reflects light off the back side of the web and responds to a printed mark thereon.
Such printers also may be adapted to permit the removal of individual labels as they are printed. The construction of the printhead may be such that the web and ribbon are advanced by the length of the inter-label gap plus a significant fraction of an inch after printing of each label and before stopping for removal of the label, in which case the web and ribbon must be backfed an equal distance before printing the next label to avoid leaving an unprintable area of the label.
The power flow to each heater element during energization is relatively constant, being determined by the supply voltage and the electrical resistance of the heater. The energy per printed dot for uniform ink transfer is a function of the web speed and the average printhead temperature. When printing individual labels, the web speed may not be constant, but may be smoothly accelerated and decelerated to allow for inertia of the mechanism. This requires changes in the energization to maintain uniform print quality across the areas printed during speed changes.
Such printers should complete the individual labels as rapidly as practical upon receipt of data therefor. Printing of a label requires three steps: receipt by the controller of a label description in a terse label-description language describing the known objects to be printed, such as text and bar codes but not the dot patterns from which they are formed; formation of the label image in a bit-map memory by the controller, where bits in the map correspond to physical dots in the image; and transfer of the dots forming the label image from bit-map to the printhead, energization of the printhead, and feeding of the web and transfer ribbon as described above.
The thermal transfer ribbon may be fed from a supply roll before printing and then taken up on a take-up spindle after use. Some prior art thermal printers use a slip clutch to maintain a tension on the ribbon take-up spindle. The slip clutch creates a constant torque output on the ribbon take-up spindle. Thus, the slip clutch does not compensate for the decrease in tension due to the increasing radius of the take-up spindle. Further disadvantages result from the use of a clutch. The clutch puts an additional load on the stepper motor, and as a result, the stepper motor must be larger and its drive circuitry must operate at higher power levels. Also, the ribbon tension is not easy to adjust using a slip clutch. Finally, changes in tension occur due to clutch wear from use unless the clutch is calibrated periodically readjusted.
Prior art printers typically have been housed in case structures which have not accounted for ease of assembly, ease of repair, and reduction in manufacturing costs. Additionally, the case structures for prior art thermal printers has not been designed optimally to accommodate typical operating environments and conditions.
For example, studies of thermal printers in the work place have disclosed that often the thermal printers are operated with a main cover in an open position in order to provide ease of access in loading and changing media as well as ribbon stock. As a result of operating the thermal printer with the main panel in the open position, the cover often may become damaged or broken off of the printer body. As such, it would be preferable to provide a case structure for a thermal printer which allows for easy removal of the main cover.
Prior art thermal printer case structures involve numerous fasteners and body members in their assembly. These case structures often were formed of stamped and formed sheet metal plates. The numerous fasteners and components in the case structure required additional time in the initial assembly as well as additional time when repairing the thermal printer. As such, it is desirable to provide a thermal printer case structure which can be quickly and easily assembled with;,as few fasteners as possible and conveniently disassembled when necessary.
Prior art thermal printers have another problem with regard to assembly and disassembly of subassemblies. The various components or subassemblies often were interrelated and interconnected. As such, when the prior art thermal printer was being assembled or repaired, additional assembly or disassembly time was required. Additionally, the prior art printers were difficult to reconfigure for a variety of printing operations due to the interconnection and interrelation of the subassemblies.
Prior art printers also have another problem with regard to the platen roller used in the device. In a printer, a platen usually includes a platen shank which defines a cylindrical platen surface. The platen shank has shaft portions projecting from either end which are typically engaged in some form of ball bearing roller assembly. The roller assembly and platen roller are attached to a frame portion of the case structure to retain the platen roller in a desired position. Because a high degree of precision is required in the position of the platen, complex snap ring washers and roller assemblies were devised to mount the platen roller in the case structure. However, such complex assemblies create difficulties in manufacturing and repair of the printer. As such, it is desirable to provide a platen roller which simplifies the mounting of the platen roller in the case structure.
As discussed above, the prior art thermal printing devices may be quite complex and burdensome in the assembly and disassembly process. The printhead assembly of the prior art thermal printers can also be quite complex and require substantial effort to assemble or repair. One form of prior art printer employs a printhead assembly which pivots about an axis which lies between the platen frame and the case structure. This arrangement provides only a single degree of freedom and hence a high precision adjustment of the printhead relative to the platen and the print medium is difficult if not impossible to achieve. In other words, the frame structure which supports the platen roller is mounted to the case structure and provides a foundation for the printhead assembly. This arrangement of the printhead limits movement of the printhead to only a pitching movement towards and away from the platen. Because the printhead's assembly is limited to one of the three degrees of motion, high precision fine adjustment of the printhead relative to the print medium can be difficult if not impossible to achieve.
Additionally, the arrangement of the printhead assembly as discussed resulted in adjustment portions of the printhead assembly being difficult to access during a printing operation. As such, adjustments to the printhead assembly must be carried out by numerous iterations of printing a desired label and stopping the machine for adjustment. Such an iterative procedure for adjustment can be quite time consuming and therefore inefficient.
Having reviewed the problems with the case structure, platen roller and printhead assembly of the prior art thermal printers, we now turn to the media delivery system or assembly and the problems found therein in prior art thermal printers. While such media delivery assemblies achieved their purpose, there are several with problems which would be desirable to overcome. The unaided removal of spent transfer ribbon from the take-up spindle is difficult, in that the ribbon is typically a very thin plastic material with a printing substance applied thereto. As the take-up spindle winds up the spent printing ribbon, the ribbon tends to wind rather tightly around the outside surface of the spindle. Additionally, the thin plastic material tends to be somewhat slippery and difficult to grip when trying to remove it from the spindle for disposal.
One prior art printer uses an empty ribbon core attached to the spindle to accumulate the spent printing ribbon. An empty core is attached to the take up spindle and the spent ribbon is wound around the empty core. When disposing of the spent ribbon, the core is slipped off of the spindle and the empty core, with the spent ribbon wound there around is disposed of. This method is problematic in that an empty core must be made available every time spent ribbon is to be accumulated. If a core is not available, ribbon could be wound around the spindle without the core, however, removal of the spent ribbon from the spindle without the core is a very difficult task.
Another way of overcoming the problem of disposing of spent ribbon is to provide a spindle which has a wire form to provide a space between the spent ribbon and the outer surface of the spindle. In this regard, a U-shaped wire form is positioned on the spindle with one leg of the U-shaped wire form extending into the spindle generally parallel with a central spindle axis and a second leg of the wire form placed on the surface of the spindle or slightly above the surface of the spindle. As ribbon is wound around the wire form on the spindle a space is created between the spent ribbon and the spindle surface. When the spent ribbon is to be disposed of, the wire form is removed from the spindle and the spent ribbon is axially slipped off of the spindle. This form of take-up spindle, however, can be problematic in that it employs loose parts and still requires the removal of a component relative to the spent ribbon. For example, the U-shaped wire form could be lost which would create the problem of winding spent ribbon around a bare spindle or replacement of the wire form. Additionally, removal of the wire form from beneath the tightly wrapped spent ribbon can be somewhat difficult and is comparable to removal of spent ribbon from a spindle without the wire form.
A problem arises in prior art printers with the consistency of back tension on the transfer ribbon. printing ribbon. This back tension is critical to the smooth flow of transfer ribbon through the media path during the printing operation. This requires that a relatively constant back tension be maintained on the ribbon supply roll during both forward feed during printing and during the back feed operation discussed above. If sufficient tension is not retained in the ribbon, or if a slack develops during back feed, the ribbon may tend to smear or mark the media adjacent to it. In this regard, some prior art printers have devised clutch mechanisms to provide back tension on the printing ribbon. However, many clutch mechanisms were rather complex requiring numerous parts for proper operation. Accordingly, numerous parts resulted in additional costs as well as assembly and repair time and effort. As such, it would be desirable to provide a simplified clutch mechanism for use with a thermal printer.
Printers are often shipped overseas, which requires that they be able to operate from 240 volt power sources. One prior art way of accommodating both 120 and 240 volt operation in the same power supply design is by use of a jumper to select the desired operating voltage. It is further desirable to build and keep printers in semi-finished form and then adapt the semi-finished unit to either 120 volt or 240 volt operation just before shipment.